The proposed research will use a DNA tumor virus, SV40, to probe the following questions: 1) what determines the initiation of genetic recombination in eukaryotic cells and 2) what is the optimal functional signal for the initiation of DNA replication. To date we have little insight into the molecular basis for viral-host interactions that lead to tumorigenesis. The specificity of the primary recombination event between viral DNA and the host chromosome is presently unknown. Mechanisms of eukaryotic recombination will be delineated by analyzing the nucleotide sequence of the initial viral-host recombinant joint formed in SV40-infected cells and by development of an in vitro subcellular recmbination system. To understand the mechanism of initiation of DNA replication in tumor viruses, we will first define the precise signal for DNA replication by constructing mutants of SV40 that contain deletions extending to the limits of the minimum nucleotide sequence required to initiate replication. During undiluted serial passage of SV40, variants with snapback regions evolve which contain multiple copies of the origin of SV40 DNA replication arranged as an inverted repetition covalently joined to cellular DNA. A detailed study of these snapback regions and their possible contribution to the replicative advantage of these SV40 variants will give us insight into their biological significance.